Another way to look at it would be to ask ourselves “How can we figure out which index we should store a value in by a string key?. So what if we instead wanted to be sure that we could access each key value pair by its key. values would be ("bar", 2) in the example above. You can only access a value in a list by the index or position that it’s stored at. ![]() But how do we store key value pairs in a list? We could store them as key value pairs, but how do we know where each pair is? Isn’t list just a sequence of values? This basically means that it all comes down to storing values in a list/array. Obviously we are not allowed to use Python dictionaries or any other data type that rely on Hash Tables, since the whole point of this exercise is to create one of our own. The first step is to initiate our class and to think about how we will achieve what we’re trying to do. We will try to emulate the behavior of how a dictionary works. In this article I will create a custom Hash Table class with Python, however this could be done in any other language. Then we’ll truly understand how things are working behind the scenes. Why does Hash Tables have constant O(1) lookup time? The best way to properly understand these things is to try to create our own implementation of a Hash Table. So what is the lookup time of a Hash Table? What is the time it takes add values? Perhaps you know the answers to those questions, but even if you do, do you really feel confident that you have a good grasp of why those answers are true? The way Python handles it in the background is hashing that key that you used and then it tries to figure out how to store the value in a traditional array. Unlike some other data structures that are rarely used in real life situations, Hash Tables are used all the time.įor example, by using a dictionary in Python like data = 1 you are actually using a hash table. You would need to read the source code of the set class (written in C) to see exactly how it works.īut the essential detail is correct: to iterate over a set potentially takes more work than iterating over a list.Coderbook How to Create a Hash Table From Scratch in Python Ī Hash Table is one of the core data structures that you need to have a good understanding of as a Software Engineer. The details as I have explained it may differ from version to version, or from one interpreter (CPython) to another (PyPy, IronPython, Jython, Stackless, etc). The exact layout of the array (the hash table) is unpredictable, and when Python tries to iterate over the elements it may have to visit each cell in turn, skipping the blanks. ![]() # [ blank | blank | 2 | blank | blank | 11 | blank | blank |īlank | 5 | blank | blank | blank | 15 | blank | 7 | ![]() The interpreter knows that there are only five values in the list, so it stops before hitting the blank cells.īut the set, being a hash table, has potentially many gaps: row_z = The blank cells at the end are so that append() is faster. Internally, the list will be an array of values, with no gaps: row_z = Your code is not searching the set, it is just iterating over the elements of the set one element at a time.
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